In the transmission of high tension energy, it is common for a first HGU D.C. network to supply a part of its energy to a second HGU D.C. network and to transmit the remaining power elsewhere. This may occur, for example, in the supply of power to a local community where a first high-voltage HGU network supplies a second HGU network of lower "high voltage". This second D.C. network is then connected to local converter substations which supply energy from the second HGU network to the A.C. voltage network of the community.
One such conventional conversion arrangement for converting high level D.C. from a first HGU network to a lower voltage HGU network is shown in FIG. 1. There, high tension lines 1, 2 supply the input +600 kv, -600 kv to a local inverter station. The input voltages are supplied to this first inverter station which includes current-director or inverter bridges 3, 4, 5 and inverter bridges 6, 7, and 8, symmetrical with respect to a common ground. Each of the inverters is coupled to a corresponding transformer, 9, 10, 11, 12, 13, and 14, which in turn is connected to a three-phase bus bar 15.
A second HGU network 24 is of similar but reversed design. That is, a three-phase bus bar 15b is connected to a reduced number of transformers 16, 17, 18 and 19 which in turn is connected to rectifier bridges 20, 21, 22 and 23. The two networks are connected by coupling switches 15a. Smoothing coils 27 may also be included as shown. The output voltages of the second network appear on lines 25 and 26 as reduced voltages +400 kv, -400 kv.
The efficiency of such a converter substation can be calculated as follows. If a current flows, for example, between terminals 1 and 2 over the bridges 3 to 8 which amounts to two-thirds of the nominal load Ia of the HGU D.C. network 24 (whose voltage is two-thirds of the voltage of the input D.C. network) and if the rated voltage of all converter bridges in V.sub.B, then the installed converter power is:
Input Network = 2.sup.. 3.sup.. V.sub.B.sup.. 2/3 I.sub.A = 4.sup.. V.sub.B.sup.. I.sub.A Network 24 = 2.sup.. 2.sup.. V.sub.B.sup.. I.sub.A = 4.sup.. V.sub.B.sup.. I.sub.A - Total = 8.sup.. V.sub.B.sup.. I.sub.A
It is desirable from a practical and economic standpoint to reduce this internal consumption of each converter as much as possible and to simplify the converter design.
It is accordingly an object of the present invention to provide a converter substation in HGU transmission which consumes comparatively little power and is of a simplified and economic design.
It is another object of the present invention to provide an HGU converter substation of simplified design for transforming a higher D.C. voltage to a lower D.C. voltage.
It is still another object of the present invention to provide an arrangement for converting a higher D.C. voltage to a lower D.C. voltage in an HGU system which utilizes a minimum number of converter bridges.
It is a still further object of the present invention to provide an arrangement for converting a higher D.C. voltage to a lower D.C. voltage which utilized a minimum number of smoothing elements.
It is also an object of the present invention to provide an HGU converter substation of simplified design which uses a minimum number of components and which consumes minimum power for transforming a lower D.C. voltage into a higher D.C. voltage.